1. Field of the Invention
The present invention relates to an image forming apparatus and method, more particularly to an image forming apparatus and method using ultraviolet-curable ink and performing tonal graduation correction for compensating variation in the optical density change of the coloring material due to difference in ultraviolet light irradiation conditions.
2. Description of the Related Art
As an image forming apparatus, an inkjet printer (inkjet recording apparatus) is known, which comprises an inkjet head having an arrangement of a plurality of nozzles and forms images on a recording medium by ejecting droplets of ink from the nozzles toward the recording medium while causing the inkjet head and the recording medium to move relatively to each other.
In particular, technology using ultraviolet-curable ink (so-called UV ink) in an inkjet type image forming apparatus is known.
For example, among image forming apparatuses aimed at achieving good ink curing properties with any type of recording speed, without involving complicated changes in the exposure conditions, there is known an image forming apparatus which forms an image by ejecting ultraviolet-curable ink toward a recording medium by means of an inkjet type of recording head and then curing and fixing the ink deposited on the recording medium by irradiating ultraviolet light from an irradiation device, and which, more particularly, reduces the maximum ejection volume of the ink when in a recording mode using a fast image recording speed, and increases the maximum ejection volume of the ink when in a recording mode using a slow image recording speed (see, for example, Japanese Patent Application Publication No. 2004-314598).
In the inkjet type image forming apparatus using UV-curable ink in the related art, even in a high-speed printing mode using a fast image recording speed, the UV irradiation energy amount that is irradiated onto the ink deposited on the recording medium is set to the same level as in a low-speed printing mode using a slow image recording speed, in order to cure the ink in the same manner as in the low-speed printing mode.
The UV irradiation energy amount is the product of the irradiation intensity and the irradiation duration, and in the case of the high-speed printing mode, since the recording medium is conveyed at high speed and the irradiation duration is shortened, then it is necessary to raise the irradiation intensity by a corresponding amount.
However, even in cases where the UV irradiation energy amount represented by the product of the irradiation intensity and the irradiation duration is the same, if the irradiation intensity is raised, then a phenomenon occurs whereby there is variation in the curing reaction of the ultraviolet-curable ink and/or the fading reaction of the coloring material. When a phenomenon of this kind occurs, then the optical density change of the coloring material varies, and a difference in the color density of the coloring material arises between the high-speed printing mode and the low-speed printing mode, for example. Consequently, depending on the printing mode, an image of the prescribed tonal graduations can not be obtained.
Furthermore, in the apparatus described in Japanese Patent Application Publication No. 2004-314598, for example, it is sought to achieve a reliable curing reaction by reducing the maximum ejection volume in the case of the high-speed recording mode and increasing the maximum ejection volume in the case of the low-speed recording mode. However, in the case of the high-speed print mode, when the irradiation intensity is increased, variation occurs in the curing reaction of the ultraviolet-curable ink and/or the fading reaction of the coloring material as described above, thus leading to an alteration in the optical density of the coloring material compared to the low-speed printing mode. Nevertheless, Japanese Patent Application Publication No. 2004-314598 makes no mention of this problem and is not able to resolve the problem.